Cold Molecules - New avenue to the 5th phase of matter

Using a method usually more suitable
to billiards than atomic physics,
researchers from Sandia and Columbia
University have created extremely cold
molecules that could be used as an
improved first step in creating molecular
Bose-Einstein condensates (BECs). BECs, predicted in the 1920s but not
actually made until 1995, represent a
special fifth phase of matter -- like
liquids, solids, gases, and plasmas -- but
can only exist at extremely low
temperature. Their existence was first
proposed by Satayendra Nath Bose, then
formalized by Albert Einstein.

Future applications, assuming BECs
can be usefully harnessed, range from
improved atomic clocks to individual
yes/no switches (Q-bits) in computers;
from precision gravity detectors that could
perhaps locate underground caverns to a
model black hole in which light can enter,
but cannot escape.

Researchers from Sandia and
Columbia made a serendipitous discovery
while studying collisional energy transfer
between a beam of atoms intersecting a
beam of molecules. They noted that some
collisions occurred -- as they might
between two billiard balls -- at exactly
the right velocity for molecules to become
motionless.

Zero Kelvin

The definition of a cold molecule is
one that is moving slowly, and this
experiment produced some very slowmoving
molecules. (Zero Kelvin, or
absolute zero, is defined as a molecule or
atom being stationary.)

The study led to a new technique for
cooling molecules to tens of milliKelvin
temperatures -- tens of thousandth of a
Kelvin. In a paper, the team reported that
single collisions among molecules in two
beams of nitric oxide molecules with
argon atoms can produce nitric oxide
molecules with speeds no greater than 15
meters per second, equal to a maximum
temperature of 0.4 degrees Kelvin.

"Our technique has promise to be
developed into a first step in the cooling
process needed for a molecular Bose-
Einstein condensate," says Sandia
researcher and principal investigator Dave
Chandler. The work is co-authored by
Sandia post-doctorate Mike Elioff and
James Valentini of Columbia University.

The main method used to
achieve atomic ultra-cooling to
the microKelvin temperature
range -- the same preliminary
cooling range as the Sandia
technique -- makes use of
laser beams that intersect at a
point. An atom, possessing the
appropriate absorption
characteristics, passing through
that point in effect stands still,
like a kid in a dodgeball game
struck from all sides with balls.
Transfixed by pressure from
the beams, the atom becomes
almost motionless.

The problem in cooling molecules by
the laser method is that while some atoms
possess characteristics that can be
harmonically matched by a laser
frequency, like the same note played by
two pianos, molecular energy frequencies
are more complex. This complexity makes
them unsuitable for this type of laser
cooling.

This leaves the field open for other
techniques to be developed for the
preliminary cooling of molecules. Four or
five other techniques, published recently,
had some level of success at cooling
molecules. The most successful method to
date has been the welding of ultracold
atoms together to make ultracold
molecules.

An open field

"We only manage to cool one
molecule in a million," says Chandler.
"But -- inefficient or efficient -- we
generate cold molecules. With some
improvements, we hope to be able to
make substantial numbers of them."

Molecules are cheap, he says, so getting
one in a million cooling collisions out of a
quadrillion total collisions per second the
molecules undergo in the beams doesn't
bother him.

This first-step method -- the only one
to rely solely on the masses of the atoms and molecules involved -- could be
useful in slowing down the speed of a
variety of molecules sufficiently such that
magnetic or electrical traps can be used to
cool molecules further. Without prior
slow-down, molecules would escape these
relatively weak traps, like molecules of
water rising from the surface of hot
coffee.

Instruments in Chandler's lab,
working at their resolution limit, show
selected molecules in the intersecting
beams slowing from 600 meters/second to
15 meters/second. The group's calculations
indicate an average speed to be on
the order of 4 meters/second. This average
speed is equivalent to a temperature in
the milliKelvin range, still several
thousandths of a degree above the
universe's absolute zero of -273 Celsius.
The last 99 yards, so to speak, are the
hardest: Bose-Einstein condensates exist
in the nanoKelvin range, still six orders of
magnitude colder.

The work, funded by DOE's Basic
Energy Sciences, focuses on understanding
how energy flows between molecules for
a better understanding of heat transfer.

The Department of Energy's Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time.